During vehicle operation, fuel vapor may be generated in the fuel system of a vehicle due to heat generated from engine operation, for example. As the amount of fuel vapor increases pressure in the fuel system may increase. In order to regulate the fuel system pressure, fuel vapor may be released from the fuel system. However, releasing fuel vapor to the atmosphere causes an increase in fuel vapor emissions. Accordingly, the fuel system may include a fuel vapor canister to filter fuel vapor upon release from the fuel system to prevent the release of fuel vapor emissions. Eventually, the fuel vapor canister may become filled with an amount of fuel vapor. The fuel canister may be cleared of fuel vapor by way of a purging operation. A fuel vapor purging operation may include opening a purge valve located in the manifold of the internal combustion engine to introduce the fuel vapor into the cylinder(s) of the internal combustion engine for combustion so that fuel economy may be maintained and fuel vapor emissions may be reduced. Thus, during a fuel vapor purge operation the internal combustion engine is required to be operating in order to provide engine vacuum to draw the fuel vapor from the fuel vapor canister into the cylinder(s) as well as to provide combustion for the fuel vapor.
In a hybrid electric vehicle, in order to achieve gains in fuel economy and powertrain efficiency the internal combustion engine may not always be operating. Accordingly, the opportunity for fuel vapor purging may be limited even though fuel vapor may still be accumulating in the fuel vapor canister.
One approach to control fuel vapor purge involves purging based on the fuel tank pressure and the amount of time since the last purge operation. Based on one or more of those indicators exceeding a threshold, the internal combustion engine may be turned on so that the fuel vapor canister may be purged. See, for example, U.S. Pat. No. 6,557,534.
However, the inventors herein have recognized that the above approach has some issues. In particular, under some conditions, the internal combustion engine may be started unnecessarily in order to perform the fuel vapor purge operation. For example, since the above approach only purges when the fuel vapor canister is substantially full hybrid electric operation may be interrupted unnecessarily in order to perform a purging operation. By turning on the internal combustion engine solely to perform a purge operation the operating efficiency of the hybrid electric vehicle may be reduced since additional fuel may be consumed to start the engine, for example. Furthermore, an increase in engine start events may reduce operating transparency to the vehicle operator.
The above issues may be addressed by, in one example, a method of controlling fuel vapor purging in a hybrid electric vehicle capable of selectively operating an internal combustion engine, the method comprising: determining a fuel tank condition parameter based on an amount of liquid fuel residing in a fuel tank of the hybrid electric vehicle and a duration since a previous fuel tank filling event; in response to the fuel tank condition parameter exceeding a threshold limit, initiating operation of the internal combustion engine and purging fuel vapor from the fuel vapor canister for a predetermined duration; and in response to the fuel tank condition parameter being less than the threshold limit, selectively purging fuel vapor from the fuel vapor canister based on an engine operating condition.
Thus, by performing fuel vapor purging based on the condition of the fuel tank, the state of the fuel vapor canister may be regulated to prevent fuel vapor emissions from being released to the atmosphere and operating the internal combustion engine only for fuel vapor purging purposes may be reduced which, in turn, may increase the operating efficiency of the hybrid vehicle. In particular, by considering the condition of the fuel tank, and more specifically, estimating an amount of transferable fuel vapor residing in the fuel tank based on the amount of liquid fuel in the tank, fuel vapor purging operations may be performed with increased precision. Further, by estimating the likelihood of a fuel tank filling event occurring based on a duration since a previous fuel tank filling event, a determination may be made to prioritize hybrid electric operation in favor of fuel vapor purging operation in order to improve operating efficiency of the hybrid electric vehicle. For example, if a fuel tank is substantially full of liquid fuel and fuel vapor purging has occurred, the fuel vapor purge frequency, rate, and/or duration may be reduced since the likelihood of a fuel tank filling event is low. As another example, as the amount of liquid in the fuel tank decreases the amount of fuel vapors residing in the fuel tank increases as well as the likelihood of the fuel tank being filled, thus, the purge frequency, rate, and/or duration may be increased in order to create space in the fuel vapor canister to handle an addition of fuel vapor entering the fuel vapor canister upon a fuel tank filling event.
Furthermore, by considering the condition of the fuel tank, the fuel vapor purge rate may be adjusted to match engine operating conditions so that the engine does not have to be adjusted strictly for fuel vapor purging purposes which may reduce operating transparency to the vehicle operator. In other words, fuel vapor purging may be performed opportunistically to meet vehicle operating conditions. In this way, fuel vapor emissions may be regulated without decreasing the operating efficiency of the hybrid vehicle and providing improved operating transparency to the vehicle operator.